This invention relates to gas turbine control technology and, more specifically, to the maintenance of high combustor temperatures in gas turbines for low emissions operation, particularly under part load or turndown conditions.
Proper operation of a Dry Low-NOx (DLN) combustion system requires a high combustor reference temperature, e.g., greater than 1675xc2x0 F. in some gas turbine models. For gas turbines that do not have variable compressor inlet guide vanes, maintaining high combustor temperature can only be achieved for turbine loading down to about 90-92% of base loadxe2x80x94via firing temperature control. At lower loads, the combustor reference temperature will decrease below the required value for optimum DLN system operation.
Each combustor in one conventional DLN combustion system is a two-stage (primary and secondary) premixed combustor designed for use with natural gas fuel and capable of operation on liquid fuel. The combustion system operates in four distinct modes:
1) Primary Fuel to the primary nozzles onlyxe2x80x94Flame is in the primary stage only. This mode of operation is used to ignite, accelerate and operate the machine over low- to mid-loads, up to a pre-selected combustion reference temperature.
2) Lean-Lean Fuel to both the primary and secondary nozzlesxe2x80x94Flame is in both the primary and secondary stages. This mode of operation is used for intermediate loads between two pre-selected combustion reference temperatures.
3) Secondary Fuel to the secondary zone onlyxe2x80x94Flame is in the secondary zone only. This mode is a transition state- between lean-lean and premix modes. This mode is necessary to extinguish the flame in the primary zone, before fuel is reintroduced into what becomes the primary premixing zone.
4) Premix Fuel to both primary and secondary nozzlesxe2x80x94Flame is in the secondary stage only. This mode of operation is achieved at and near the combustion-reference-temperature design point. Optimum emissions are generated in the premix mode.
The load range associated with these modes varies, but generally, the premix operating range is 50 percent to 100 percent load.
Currently, variable inlet guide vanes and inlet bleed heat are used to maintain gas turbine combustor temperature in a range for low emissions operation. There remains a need, however, for a control mechanism that will maintain high combustor reference temperature for DLN machines that do not have variable inlet guide vanes operating at part load conditions and thereby also maintain emissions requirements.
In the exemplary embodiment of this invention, up to 20-22% of the compressor discharge air is bled off, thereby reducing the air available for combustion. This forces the turbine firing temperature to increase which allows the DLN combustion system to operate in its optimum premix mode. Operation in the premix mode keeps NOx at its lowest rated value.
More specifically, air is bled from the compressor discharge section uniformly around the circumference of the machine using up to 14 extraction locations. This allows uniform flow distribution within the combustion system. The amount of air extracted varies with several ambient and machine parameters. Control of the extraction air is via a stop and control valve, and the extracted air is bled into the gas turbine exhaust stack (or to any other area or system).
In the exemplary embodiment, five separate control modes, implemented by five software modules, provide input to a minimum value gate. One mode determines an input value based on process parameters that include compressor pressure ratio (CPR); turbine rotor speed; ambient temperature and a valve command reference value. A second mode provides an input value based on process parameters that include CPR; turbine exhaust gas temperature; and a valve command reference value. A third mode provides an input value based on temperature corrected turbine rotor speed and a valve command reference value. Fourth and fifth inputs provide input values based on mechanically set overrides.
The various input values to the minimum value gate represent a required valve stroke of the control valve.
The minimum value gate will select the input that reflects minimum valve stroke and hence the minimum amount of compressor discharge air that can be bled off according to the requirements of the particular software module in control.
The output signal from the minimum value gate will be input to an actuator system for the control valve that bleeds air from the compressor discharge air. The associated valve is employed to halt the extraction of bleed air in the event of faulty operation of the control valve.
The compressor air bleed system is designed to extract enough air to keep the machine firing temperature at a predetermined high value to keep the combustors operating in the premix mode. The system continuously modulates the air bleed control valve, thus allowing stable and optimum DLN combustion under loading varying from the rated base load (100%) value down to 65% of the base load value.
Accordingly, in its broader aspects, the invention relates to a method of operating a combustor in a gas turbine that receives combustion air from a compressor comprising a) controlling combustion temperature as a function of compressor discharge air bled from the compressor by determining a minimum amount of compressor discharge air required for maintaining a predetermined combustion temperature; and b) bleeding compressor discharge air in excess of the minimum amount from the compressor via a control valve.
In another aspect, the invention relates to method of operating a combustor in a gas turbine that receives combustion air from a compressor comprising: a) controlling combustion temperature as a function of compressor discharge air bled from the compressor by determining a minimum amount of compressor discharge air required for maintaining a predetermined minimum combustion temperature; and b) bleeding compressor discharge air in excess of the minimum from the compressor via a control valve; wherein step a) is carried out by using one or more process parameters to determine a bleed valve temperature reference and using that reference to determine required bleed flow, subject to overrides from other software modules acting through a minimum value selection function that compares multiple inputs, each based on one or more process parameters, and selecting that one of the multiple inputs that provides for the minimum amount of compressor discharge air; and wherein step b) includes bleeding air from multiple extraction locations spaced circumferentially about the compressor.
The invention will now be described in connection with the drawing figures identified below.